1. Field of the Invention
The present invention relates to a supporting plate for a photomask in an apparatus for making a microchip, comprising a rectangular plate made from glass or glass-ceramic material, which is provided with a receptacle in the plate upper side for holding the photomask in operation in an accurate position and/or which has a plurality of recesses in the plate lower side for weight reduction and at the same time for increasing the stiffness of the supporting plate.
2. Prior Art
An apparatus for microchip manufacture according to the state of the art is shown in FIG. 3A Monochromatic light issues from an unshown light source. The beam is guided by two mirror surfaces 2a and 2b to a photomask 3, which is held by a supporting plate 10 in a predetermined position, which comprises a rectangular plate 4 made from glass or glass-ceramic material . A prior art or conventional supporting plate 10 used in the apparatus of FIG. 3A is shown in FIGS. 3b and 3c. The supporting plate 10 is the subject of the p resent invention.
The monochromatic light rays are combined by a lens system 5 and form a predetermined conductor strip structure on a wafer 6 mounted on the wafer plate 7 by means of the photomask 3, which is later etched away in a later etching process. A number of the same type of integrated circuits are produced in the wafer 6 which acts as substrate, which are separated to form the semiconductor microchip after testing their operability.
As shown in FIGS. 3b and 3c the photomask 3 is received in a circular receptacle 8 and is held in this position by low pressure or vacuum. According to the state of the art the photomasks 3 typically have a size of 6 inches (square shape with an edge length of about 152 mm, thickness of about 9 mm). The size of the photomask is directly related to the size of the receptacle 8; at the corners the spacing a of the photomask 3 from the inner side of the receptacle 8 must amount to at least 10 mm.
The photomask can be held in a metal support with a suitable central opening according to the disclosure in German Patent Document DE 88 00 272 U1. By means of this type of support it is possible to accurately position the photomask in a simple manner in the receptacle or hole in the supporting plate with the help of centering pins and a locking device and to perform the work through the central opening. Because of that an automatic guidance of the photomask in the supporting plate is not only possible, but also an automatic and simple manipulation of the support with the photomask between individual stations is possible. The support has triangular recesses formed in the corner regions for weight reduction without damaging stiffness.
During the exposure of the wafer 6 motions of the supporting plate 10 for the photomask 3 and also the wafer plate 7 are performed. The supporting plate 10 of the photomask is guided so that its motions are exclusively in the X-direction, while the wafer plate 7 moves in the X-direction and Y-direction.
In the embodiment according to the state of the art acceleration of the supporting plate for the photomask 3 is on the order of 3 g (about 30 m/sec.sup.2). Two fundamental requirements for the structure of the supporting plate 10 for the photomask 3 result because of the acceleration amplitudes. There must be a reduction or minimization of the mass moved, on the one hand, and, on the other hand, an increase in the dynamic stiffness. Eigenvibrations of the system that produce structural errors in the wafer are excited by the acceleration amplitudes.
During the exposure of the wafer 6 the motions of the supporting plate 10 for the photomask 3 are monitored and the position deviations are compensated by adjusting elements. There are three actuators for the Z-coordinate, which are located in the recesses 9a, 9b and 9c of the supporting plate 10. A Lorentz motor 100 is provided for compensation in the X-direction, which is attached to the front edge fe of the supporting plate 10, and of course on the edge on which the recesses 9a and 9b for two Z-actuators are provided.
An interferometer mirror on one of the long side surfaces provides a measure for the positioning variable for the supporting table for the photomask.
For example, if a supporting plate for a photomask has dimensions of 560.times.450.times.66 mm and is made from a block (e.g. the glass material Zerodur.RTM., with a density of 250 kg/m.sup.3), except for the holes for the photomask and the Z-actuators, it has a total mass of about 30 kg. The requirements regarding mass for a suitable acceleration however require a mass between 10 and 14 kg.
Pockets 11 are cut from the complete material according to the state of the art. One possible embodiment is shown in FIGS. 4A and 4b. In FIG. 4a a top view of the prior art supporting plate 10 with the recesses 8 for receiving the photomask 3 is shown, with a series of pockets 11 and 11' which are formed as blind holes that are produced in both long sides ls of the supporting plate.
In FIG. 4b a bottom view of the prior art supporting plate 10 is shown, with pockets 11" in the form of blind holes, which are distributed according to the stiffness view point. The form of the pockets can be rectangular, honeycomb-shaped or triangular. To increase the stiffness of the supporting plate each pocket can be provided with an undercut. The working direction is limited to the thickness direction (similar to the holes for the Z-actuators). The minimum wall thickness is in the range of from 4 to 5 mm. Smaller wall thickness cannot be obtained with conventional cutting operations because of the cutting forces and the brittleness of the glass or glass-ceramic material. The supporting plate for a 6-inch photomask can be made with a mass of about 14 kg with these steps.
It is of special importance for operations that the dynamic stiffness is sufficiently high so that undesirable vibrations do not occur during the exposure process.
Manufacturers of apparatus for microchip manufacture require a frequency of at least 500 Hz for the bending vibration of the total system.
The eigenfrequencies obtained with the prior art structure are on the order of about 450 Hz for bending. These values are for a glass block without added structures (e.g. Z-motor, Lorentz motor) in free-free position.
Changes of the pocket size in both edge regions of the supporting plate for the photomask have shown that a significant increase in the dynamic stiffness it is not possible. The bending eigenfrequencies are in a range between 700.+-.30 Hz. Comparable experimental vibration experiments have shown that a very good correlation between calculated and measured variables is produced.
A change in the pocket position, as experiments have shown, leads to no noteworthy increase in the dynamic stiffness.
The known construction measures for weight reduction and stiffening of the supporting plate for the photomask limit the dimensions of the supporting plate so that according to the state of the art it is only possible to make a supporting plate for a 6 inch photomask.